chapter 7 sql huang xuehua. aggregate functions include count, sum, max, min, and avg query 15: find...

Chapter 7

SQLHUANG XUEHUA

AGGREGATE FUNCTIONS

Include COUNT, SUM, MAX, MIN, and AVG Query 15: Find the maximum salary, the minimum salary, and the

average salary among all employees.

Q15: SELECT MAX(SALARY), MIN(SALARY), AVG(SALARY)

FROM EMPLOYEE

Some SQL implementations may not allow more than one function in the SELECT-clause

AGGREGATE FUNCTIONS

Query 16: Find the maximum salary, the minimum salary, and the average salary among employees who work for the 'Research' department.

Q16: SELECT MAX(SALARY), MIN(SALARY), AVG(SALARY)FROM EMPLOYEE, DEPARTMENTWHERE DNO=DNUMBER AND

DNAME='Research'

AGGREGATE FUNCTIONS

Queries 17 and 18: Retrieve the total number of employees in the company (Q17), and the number of employees in the 'Research' department (Q18).

Q17: SELECT COUNT (*)FROM EMPLOYEE

Q18: SELECT COUNT (*)FROM EMPLOYEE, DEPARTMENTWHERE DNO=DNUMBER AND

DNAME='Research’

GROUPING

In many cases, we want to apply the aggregate functions to subgroups of tuples in a relation

Each subgroup of tuples consists of the set of tuples that have the same value for the grouping attribute(s)

The function is applied to each subgroup independently SQL has a GROUP BY-clause for specifying the grouping

attributes, which must also appear in the SELECT-clause

GROUPING Query 20: For each department, retrieve the department number, the

number of employees in the department, and their average salary.

Q20: SELECT DNO, COUNT (*), AVG (SALARY)FROM EMPLOYEEGROUP BY DNO

In Q20, the EMPLOYEE tuples are divided into groups--each group having the same value for the grouping attribute DNO

The COUNT and AVG functions are applied to each such group of tuples separately

The SELECT-clause includes only the grouping attribute and the functions to be applied on each group of tuples

A join condition can be used in conjunction with grouping

GROUPING

Query 21: For each project, retrieve the project number, project name, and the number of employees who work on that project.

Q21: SELECT PNUMBER, PNAME, COUNT (*)FROM PROJECT, WORKS_ONWHERE PNUMBER=PNOGROUP BY PNUMBER, PNAME

In this case, the grouping and functions are applied after the joining of the two relations

This type of selection is very useful.

THE HAVING-CLAUSE

Sometimes we want to retrieve the values of these functions for only those groups that satisfy certain conditions

The HAVING-clause is used for specifying a selection condition on groups (rather than on individual tuples)

THE HAVING-CLAUSE

Query 22: For each project on which more than two employees work , retrieve the project number, project name, and the number of employees who work on that project.

Q22: SELECT PNUMBER, PNAME, COUNT (*)FROM PROJECT, WORKS_ONWHERE PNUMBER=PNOGROUP BY PNUMBER, PNAMEHAVING COUNT (*) > 2

So what is the difference between WHERE and HAVING? WHERE : before ; HAVING : after

ORDER BY

The ORDER BY clause is used to sort the tuples in a query result based on the values of some attribute(s)

Query 28: Retrieve a list of employees and the projects each works in, ordered by the employee's department, and within each department ordered alphabetically by employee last name.

Q28: SELECT DNAME, LNAME, FNAME, PNAME FROM DEPARTMENT, EMPLOYEE,

WORKS_ON, PROJECTWHERE DNUMBER=DNO AND SSN=ESSN AND PNO=PNUMBERORDER BY DNAME, LNAME

ORDER BY

The default order is in ascending order of values We can specify the keyword DESC if we want a

descending order; the keyword ASC can be used to explicitly specify ascending order, even though it is the default

Specifying Updates in SQL

There are three SQL commands to modify the database; INSERT, DELETE, and UPDATE

INSERT

In its simplest form, it is used to add one or more tuples to a relation

Attribute values should be listed in the same order as the attributes were specified in the CREATE TABLE command

INSERT

Example:

U1: INSERT INTO EMPLOYEEVALUES ('Richard','K','Marini', '653298653', '30-DEC-52','98 Oak Forest,Katy,TX', 'M', 37000,'987654321', 4 )

An alternate form of INSERT specifies explicitly the attribute names that correspond to the values in the new tuple

Attributes with NULL values can be left out Example: Insert a tuple for a new EMPLOYEE for whom we only know the

FNAME, LNAME, and SSN attributes.

U1A: INSERT INTO EMPLOYEE (FNAME, LNAME, SSN) VALUES ('Richard', 'Marini', '653298653')

INSERT

Important Note: Only the constraints specified in the DDL commands are automatically enforced by the DBMS when updates are applied to the database

Another variation of INSERT allows insertion of multiple tuples resulting from a query into a relation

INSERT Example: Suppose we want to create a temporary table that has the

name, number of employees, and total salaries for each department. A table DEPTS_INFO is created by U3A, and is loaded with the summary information retrieved from the database by the query in U3B.

U3A: CREATE TABLE DEPTS_INFO(DEPT_NAME VARCHAR(10), NO_OF_EMPSINTEGER, TOTAL_SAL INTEGER);

U3B: INSERT INTO DEPTS_INFO (DEPT_NAME, NO_OF_EMPS, TOTAL_SAL)

SELECT DNAME, COUNT (*), SUM (SALARY)

FROM DEPARTMENT, EMPLOYEEWHERE DNUMBER=DNOGROUP BY DNAME ;

INSERT

Note: The DEPTS_INFO table may not be up-to-date if we change the tuples in either the DEPARTMENT or the EMPLOYEE relations after issuing U3B. We have to create a view (see later) to keep such a table up to date.

DELETE

Removes tuples from a relation Includes a WHERE-clause to select the tuples to be deleted Tuples are deleted from only one table at a time (unless CASCADE is

specified on a referential integrity constraint) A missing WHERE-clause specifies that all tuples in the relation are to

be deleted; the table then becomes an empty table The number of tuples deleted depends on the number of tuples in the

relation that satisfy the WHERE-clause Referential integrity should be enforced

DELETE

Examples:U4A: DELETE FROM EMPLOYEE

WHERE LNAME='Brown’

U4B: DELETE FROM EMPLOYEEWHERE SSN='123456789’

U4C: DELETE FROM EMPLOYEEWHERE DNO IN (SELECT DNUMBER

FROM DEPARTMENTWHERE DNAME='Research')

U4D: DELETE FROM EMPLOYEE

UPDATE

Used to modify attribute values of one or more selected tuples

A WHERE-clause selects the tuples to be modified An additional SET-clause specifies the attributes to be

modified and their new values Each command modifies tuples in the same relation Referential integrity should be enforced

UPDATE

Example: Change the location and controlling department number of project number 10 to 'Bellaire' and 5, respectively.

U5: UPDATE PROJECT

SETPLOCATION = 'Bellaire', DNUM = 5

WHEREPNUMBER=10

UPDATE

Example: Give all employees in the 'Research' department a 10% raise in salary.

U6: UPDATE EMPLOYEESET SALARY = SALARY *1.1WHERE DNO IN (SELECT DNUMBER

FROM DEPARTMENT WHERE DNAME='Research')

In this request, the modified SALARY value depends on the original SALARY value in each tuple

The reference to the SALARY attribute on the right of = refers to the old SALARY value before modification

The reference to the SALARY attribute on the left of = refers to the new SALARY value after modification